# ...

use Test::More;
BEGIN {
    if (eval 'use Test::Warn; 1') {
        Test::More->import(tests => 42);
    } else {
        Test::More->import(skip_all => 'Test::Warn is not available');
        # this line won't be reached, see Test::More
    }
}

warning_like { warn "foo" } qr/foo/; 

# ...